Friday 18th June 2010

Tracks, cracks and pteropods! (or where have all the sponges gone?)

Now we are (relatively) safely ensconced at the southwest ECOMAR station we can get our first look at the seabed below and more importantly the animals living there. A combination of tumultuous terrain and poor weather on previous cruises has resulted in us having absolutely no samples from this region….so it is now even more of a voyage of discovery!

Descending to the seabed we expected Isis to find some similar seabed structures to those we had seen in the northern sites. Bathymetric data indicates that here in the south we are again surrounded by basalt cliffs and terraces interspersed with soft sedimented valleys. However, a dramatically different sight awaited us on arrival at the seafloor.

The flat areas of sediment look dramatically different to those in the north, gone were the long, languid meandering trails of echinoderms about their business…replaced with large ripples and riffles in the seabed, shaped by the passage of strong currents. In the hollows between these structures the seabed was carpeted with never-ending swathes of pteropod shells. The pteropod is a pelagic mollusc found high up in the water column, far from the seabed. Its shell has become much reduced and is located internally within the soft body parts of the animal. These pteropods drift and swim on the ocean currents feeding on smaller members of the plankton. However, as with all things in the sea when they die they will sink to the seafloor (getting to be a common theme isn’t it? Whales, algae, bits of Thom’s mackerel!) and over time these pteropod shells contribute to the biogenic oozes that make up large parts of our ocean floors. The shells of pteropods contain large amounts of aragonite and carbonate, however, these deposits are often rare, compared to the biogenic oozes made up of planktonic foraminiferans, as the aragonite dissolves much more readily than the calcite of foraminiferan shells.

pteropod shells

Pteropod Clione limacina

As Isis traversed her way across the seafloor we saw huge cracks, hollows and mounds in the seabed, often full of fluffy marine snow just waiting to be devoured by a hungry sea-cucumber! It is unclear what could have made these massive earthworks, feeding fish? busy shrimps tunnelling away? What was clear was that we are dealing with something completely different to the habitats north of the Charlie-Gibbs Fracture Zone.

Surely the cliffs shouldn’t spring too many surprises? Same depth, similar topography, lots of hard substrate……

southwest cliffs

The first thing we became acutely aware of flying along the cliff faces of the southwest station was, well nothing really….in that it all looked fairly barren. Where were all the crinoids, soft corals and sponge gardens we had got so used to in the north? They had been replaced with tumbling boulders and outcrops of ancient pillow lava, sediment-filled gullies and the odd whip coral every now and then. Occasionally we may bump into a familiar funnel sponge or golden crinoids but long sections of cliff wall appeared devoid of life…even the intrepid mountaineering sea-cucumbers were missing from the sedimented screes and ledges.

northwest cliffs

So, much to ponder on as we progress to more dives to collect what characteristic species we can find and all on board now wait with baited breath for the first use in anger of Marsh’s fabled pelagic D-samplers….ctenophores and appendicularians beware!!

 Ben Wigham

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